Most conventional manual pipettes have a plunger button on the top of the handle. The plunger button is depressed by the user's thumb to manually lower a plunger shaft which in turn lowers a pipetting piston. A disposable pipette tip is mounted on a fitting attached to the lower portion of the pipettor. The seal around the pipetting piston causes suction in the disposable pipette tip when the piston is retracted. To aspirate liquid into the disposable pipette tip, the end of the tip is submerged in the liquid and, the user releases the plunger. A piston return spring causes the piston to retract thereby causing suction within the pipette tip to aspirate the liquid into the tip. The user then moves the pipette to a dispensing location and again depresses the plunger against the force of the spring in order to dispense the liquid from the pipette tip. Most manual pipettes also include a blowout spring mechanism that enables the plunger to move downward past the natural fully depressed range for aspiration in order to blow out residual liquid when dispensing from the tip.
The piston spring return force is normally selected by the manufacturer to be as light as possible, yet sufficient to reliably overcome friction associated with the piston seal arrangement and possibly additional friction associated with the return of the piston and plunger to the home position. In some cases, however, such a light spring force is not desirable. For example, when pipetting viscous fluids one may require greater spring return forces in order to ensure accurate and efficient liquid transfer. As another example, some users may find a pipette with a higher return spring force to be more ergonomic or comfortable to use even with non-viscous fluids.